1. Field of the Invention
This invention relates to the field of automatic transmission controls. More particularly it pertains to solenoid-actuated hydraulic control circuitry for selectively engaging and releasing a brake servo and friction clutch in an automatic tranmission.
2. Description of the Prior Art
In an automatic transmission brake bands are used to hold elements of a gearset against rotation by frictionally engaging the outer surface of a brake drum to which a gearset element is driveably connected. The brake band is applied and released through operation of a brake servo, an hydraulic cylinder supplied with pressurized hydraulic fluid and containing a double acting piston connected to the brake band. It is conventional practice that the same pressure be applied to the apply-side of the servo piston and to the release-side of the servo piston. The effective area of the release-side of the piston is somewhat larger than that of the apply-side.
An alternate conventional servo, in which the piston areas are of equal size, includes a valve that reduces line pressure to the apply-side in relation to the pressure at the release-side of the piston. These techniques are employed to ensure that the servo will release the brake band yet have torque capacity sufficient to hold the brake without slipping.
In certain applications in an automatic transmission, operation of one friction element such as a brake band must be synchronized with the application and release of a clutch such that neither of the friction elements is concurrently applied and torque carried by the off-going friction element is transferred smoothly to the on-coming friction element. A technique for producing this effect is to regulate the flow of hydraulic fluid from a pressure source to the friction elements and from the friction elements to exhaust by directing the flow through orifices of predetermined sizes selected to coordinate friction element operation. In the prior art an arrangement of check valves and orifices has been developed to direct fluid through one orifice while a friction element is being engaged and to direct fluid through a different orifice when the friction element is being vented.